Such first sheets, which are also sold under the name "CB forms," are known. The trend toward making such first sheets of constantly lower weight per unit area has brought the result that the opacity of such papers leaves something to be desired. Particularly when the microsphere coated back of the first sheet is also imprinted, the printing is visible through the thin paper and spoils the appearance of the front side.
Attempts have been made to improve the opacity of such papers by increasing the filler content in the first sheet and also by putting a nonreactive coating on the front. But it has been found that conventional coatings can improve the opacity and printability of the front side, but the easy divisibility required in copysets is lost.
By "easy divisibility" the following is meant. A copyset consists as a rule of a first sheet (CB), one or more middle sheets (CFB) and a back sheet (CF). For use, several of these sets are assembled into a block and cemented together at one narrow side. The adhesion of the glue for proper separation of one set at a time from the block, between the CF back and the CB top, must be less than it is between the CB bottom and CFB top and the CFB bottom and CF top. In the case of uncoated-surface first sheets the problem was solved by adjustment by means of surface preparation or by controlling the absorptiveness of the backs. Such measures failed, however, in the case of first sheets whose front surface was coated with pigments.
EP-A-0 274 886 proposed to provide the coated front additionally with a synthetic, reactive size or with a coating-texturing agent or with both agents, in order to assure easy divisibility.
A disadvantage of this proposal is that synthetic reactive sizes require, for complete curing, a certain time of exposure to a temperature range above 100.degree. C. If this is not provided, e.g., when the coating unit runs too fast, or if the drying capacity of the coating unit constitutes a bottleneck, the synthetic size does not cure, with the result that the moisture repellency or adhesive repellency necessary for the easy-division adhesive on the CB sheet provided with an imprintable front surface coating is not achieved.